Compound semiconductor epitaxial substrate and method for manufacturing same

ABSTRACT

A compound semiconductor epitaxial substrate having a pseudomorphic high electron mobility field effect transistor structure which comprises an InGaAs layer as a channel layer  9  and an InGaP layer containing n-type impurities as a front side electron supplying layer  12 , wherein an electron mobility in the InGaAs layer at room temperature (300 K) has become 8000 cm 2 /V·s or more by growing an epitaxial substrate having a pseudomorphic HEMT structure with an In composition of the channel layer  9  increased. Front side spacer layers  10  and  11  between the channel layer  9  and the front side electron supplying layer  12  may also be InGaP layers.

TECHNICAL FIELD

The present invention relates to a compound semiconductor epitaxialsubstrate used for a high electron mobility transistor using a III-Vcompound semiconductor, and a method for manufacturing the same.

BACKGROUND ART

Recently, there has dramatically been developed an electronic devicewhich utilizes a III-V compound semiconductor mainly based on GaAs,taking advantage of features thereof such as abilities to operate at aultra-high speed and at a higher frequency, and has still advancedsteadily. When fabricating the electronic device which utilizes acompound semiconductor, a thin film crystalline layer with necessaryproperties has conventionally been fabricated on a single crystalsubstrate by means of various procedures such as an ion implantationmethod, a diffusion method, or an epitaxial growth method. The epitaxialgrowth method, among the above described various methods, has widelybeen used since it has become possible not only to control an impurityamount but also to control crystal composition or thickness in anextremely wide range and with precision.

Although procedures such as a liquid phase method, a vapor phase method,and a molecular beam epitaxy (referred to as a MBE method, herein after)which is one of the vacuum deposition methods have been known as theepitaxial growth method used for such purposes as described above, thevapor phase method has commercially and widely been used because of itsability to process a large amount of substrates with highcontrollability. Especially, an metalorganic pyrolysis method (referredto as a MOCVD method, hereinafter), in which an organometallic compoundor a hydride of atomic species constituting an epitaxial layer is usedas a source material and is pyrolyzed on the substrate to grow a crystalthereof, has recently been used widely since this method is applicableto a wide range of substances and is suitable for controlling thecrystal composition and thickness with precision.

Based on the development of such manufacturing techniques as describedabove, there have recently been made various attempts to improvecharacteristics of a high electron mobility field effect transistor(referred to as a HEMT, hereinafter) which attracts attentions becauseof its usefulness as an important component of a high-frequencycommunication instrument. The HEMT is also referred to as a highelectron mobility transistor, a modulation doped field effect transistor(MODFET), or a hetero-junction field effect transistor (HJFET), and anepitaxial structure used for the HEMT is characterized in that anelectron supplying layer for supplying electrons and a channel layerthrough which electrons travel are separated from each other and playrespective roles, and that a two-dimensional electron gas accumulated inthe channel layer has a high electron mobility. An epitaxial substrateused for manufacturing the HEMT can be fabricated by employing a MOCVDmethod such that each of crystalline layers of GaAs and AlGaAs havingnecessary electron characteristics is laminated and grown on a GaAssubstrate to obtain a required structure.

Although GaAs and AlGaAs materials have widely been used as materialsfor fabricating the above described devices since these materials withany compositions can match the lattice constants thereof with each otherand allow for producing various hetero junctions while keeping goodcrystallinity thereof, it is also possible to grow a crystalline layerof InGaP by selecting an In composition such that the lattice constantthereof matches with that of GaAs. In this case, InGaP beinglattice-matched with GaAs is known to have an In composition of 0.482 to0.483 and a Ga composition of 0.518 to 0.517.

As for a III-V compound semiconductor material, In_(x)Ga_((1-x))As(wherein 0<x<1) is extremely suitable as a hetero junction material formanufacturing the HEMT, because In_(x)Ga_((1-x))As is excellent in itselectron transporting characteristic and is also capable ofsignificantly changing its energy gap in accordance with the Incomposition. However, the In_(x)Ga_((1-x))As cannot be lattice-matchedwith GaAs, so that it has conventionally been impossible to obtain anepitaxial substrate for the HEMT with significant physical properties byusing a In_(x)Ga_((1-x))As layer.

Based on the subsequent development of techniques, it has been foundthat a reliable hetero junction can be formed without unfavorablyinducing a decrease in crystallinity such as producing a dislocationeven when a material with lattice misfit is used provided that themisfit is within a limit of elastic deformation, so that there has beenmade an attempt to practically use an epitaxial substrate which utilizesIn_(x)Ga_((1-x))As as a hetero junction material. Such limit values inthe lattice misfit material are given as a function of composition andlayer thickness, and in a material based on a InGaAs layer with respectto a GaAs layer for example, the limit value has theoretically beenknown to be represented by an equation as described in J. CrystalGrowth, 27 (1974) p. 118 and in J. Crystal Growth, 32 (1974) p. 265, andthis theoretical equation is known to be experimentally correct as awhole.

Thus, even in the case of an epitaxial substrate of a HEMT structurewhich utilizes a GaAs substrate, it has become possible to manufacturean epitaxial substrate having an InGaAs layer as a part thereof by usinga strain layer within certain ranges of composition and layer thickness.For example, under the condition of usual crystal growth, it is possibleto epitaxially grow an In_(x)Ga_((1-x))As layer in which x=0.20 andwhose layer thickness is about 13 nm without inducing a decrease in itscrystallinity, and consequently, an epitaxial substrate including suchan In_(x)Ga_((1-x))As layer as a channel layer part of the conventionalHEMT through which two-dimensional electrons flow is utilized forfabricating an electronic device which has a higher mobility and isexcellent in a noise characteristic compared with the conventionaldevice.

The HEMT, in which In_(x)Ga_((1-x))As as a strain layer is used for thechannel layer part through which two-dimensional electrons flow, isreferred to as a pseudomorphic high electron mobility field effecttransistor (a pseudomorphic-HEMT) (hereinafter, referred to as apseuodomorphic-HEMT).

In addition, as described above, InGaP can be lattice-matched with GaAsprovided that an In composition is selected, and therefore, in thephseudomorphic-HEMT, an InGaP layer can be epitaxially grown as anelectron supplying layer or spacer layer thereof instead of using anAlGaAs layer. InGaP provides a high-performance HEMT, because InGaP hasadvantages that impurities are hardly incorporated therein during theepitaxial growth and the crystal purity can be favorably maintainedcompared with AlGaAs, and that a deep level referred to as a DX centeris never created when silicon is doped during the formation of an n-typelayer as in the case of AlGaAs. In addition, there has been reportedthat InGaP is advantageously used for manufacturing an electron devicebecause the InGaP has a larger energy gap and a lower surface levelcompared with AlGaAs.

When various epitaxial growths are performed in order to form apseudomorphic-HEMT structure including an InGaP layer and an InGaAsstrain layer on a GaAs substrate, the crystal growth has to becontrolled so as to precisely control a thickness of a thin crystallinelayer to be formed on the order of several nanometers, however, as aresult of recent technical improvements, an MBE method being excellentin the layer thickness controllability as well as an MOCVD method beingexcellent in the mass productivity can control the layer thickness withhigh precision, and consequently, it is now possible to obtain anepitaxial substrate of the HEMT having substantially favorablecharacteristics.

As described above, when an InGaP layer is used for an electronsupplying layer or an electron supplying layer and a spacer layer of thepseudomorphic HEMT structure, it has been found to be difficult toefficiently confine two-dimensional electrons generated from theelectron supplying layer to the InGaAs channel layer, although it ispossible to achieve improvements in characteristic of the electronicdevice such as a temperature characteristic. Thus, it has been difficultto improve a current value of the electronic device by increasing thetwo-dimensional electron gas concentration or to reduce a transientresistance of the electron device by increasing the electron mobility.

The reason thereof has been considered that an energy band profile ofInGaP is different from that of AlGaAs, that is, there is no differencebetween a position of a conduction band of the energy band structure ofGaAs and that of InGaP. If there is no difference between thesepositions of the conduction bands, electrons generated from the electronsupplying layer can not be efficiently confined to the InGaAs channellayer, and consequently, reductions in the two-dimensional electron gasconcentration and in the electron mobility would be induced. As thepreventative measures against such problems, Japanese Patent No. 3224437discloses a constitution which improves the two-dimensional electron gasconcentration and the electron mobility by inserting a strain InGaPspace layer between a channel layer and an InGaP electron supplyinglayer in order to make a difference between the positions of conductionbands. In addition, Japanese Patent No. 2994863 discloses a constitutionwhich improves the two-dimensional electron gas concentration and theelectron mobility by inserting an AlGaAs spacer layer between a channellayer and an InGaP electron supplying layer.

However, comparing with a result reported with respect to an epitaxialsubstrate used for the conventional AlGaAs pseudomorphic HEMT structurein which an InGaAs layer is used as a channel layer, an n-AlGaAs layeris used as an electron supplying layer, and an i-AlGaAs layer is used asa spacer layer between the channel layer and the electron supplyinglayer, both constitutions disclosed in the above described JapanesePatent No. 3224437 and Japanese Patent No. 2994863 have not yet achievedsatisfactory electron mobilities, in view of the capability of theepitaxial substrate used for the pseudomorphic-HEMT structure which canmake characteristics of the electronic device favorable by increasingeach value of the two-dimensional electron gas concentration and theelectron mobility.

For example, further improvements are desired when the pseudomorphicHEMT structure epitaxial substrate is used for various portableinstruments such as cellphones, because an on-resistance can bedecreased by further improving the electron mobility and thus it ispossible to reduce power consumption. In addition, further improvementsin the electron mobility are also desired in view of capabilities toreduce a calorific value by lowering the power consumption and tominiaturize of an apparatus by further increasing a degree ofintegration. Thus, in the epitaxial substrate for pseudomorphic-HEMTstructure which uses InGaP as an electron supplying layer or as anelectron supplying layer and a spacer layer, a further improvedepitaxial substrate is strongly desired which has a highertwo-dimensional electron gas concentration together with a higherelectron mobility compared with the currently reported results.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a compoundsemiconductor epitaxial substrate used for a pseudomorphic HEMT having ahigh electron mobility characteristic and a method for manufacturing thesame, which can satisfy various requirements as described above.

The present inventors have devoted ourselves to solve the abovedescribed problems, and have consequently found that a difference can bemade between a position of a conduction band of the energy bandstructure of an InGaAs layer and that of an InGaP layer by forming anepitaxial substrate of a pseudomorphic-HEMT structure with an Incomposition of the InGaAs channel layer being increased, and thus it ispossible to form an epitaxial substrate having a higher electronmobility in combination with a higher two-dimensional gas concentrationwhich have never been reported before, and have now achieved the presentinvention based on such findings.

According to a first aspect of the present invention, there is provideda compound semiconductor epitaxial substrate used for a pseudomorphichigh electron mobility field effect transistor, comprising an InGaAslayer as a channel layer and an InGaP layer containing n-type impuritiesas a front side electron supplying layer, the InGaAs layer having anelectron mobility at room temperature (300 K) of 8000 cm²/V·s or more. Atwo-dimensional electron gas concentration in the above describedchannel layer at room temperature (300 K) may be 1.80×10¹²/cm² or more.

According to a second aspect of the present invention, there is providedthe compound semiconductor epitaxial substrate as described in the abovefirst aspect, further comprising an InGaP layer as a front side spacerlayer between the above described channel layer and the above describedfront side electron supplying layer. A two-dimensional electron gasconcentration in the above-described channel layer at room temperature(300 K) may be 1.80×10¹²/cm² or more.

According to a third aspect of the present invention, there is providedthe compound semiconductor epitaxial substrate as described in the abovesecond aspect, further comprising an InGaP layer containing n-typeimpurities as a back side electron supplying layer and comprising anInGaP layer as a back side spacer layer between the above describedchannel layer and the above described back side electron supplyinglayer. A two-dimensional electron gas concentration in the abovedescribed channel layer at room temperature (300 K) may be 1.80×10¹²/cm²or more.

According to a fourth aspect of the present invention, there is providedthe compound semiconductor epitaxial substrate as described in the abovefirst, second, or third aspect, wherein an In composition of the InGaAslayer constituting of the above described channel layer is 0.25 or more.

According to a fifth aspect of the present invention, there is providedthe compound semiconductor epitaxial substrate as described in the abovefirst, second, or third aspect, wherein GaAs layers each of which has athickness of 4 nm or more are laminated on the above described channellayer respectively in contact with a top surface and a bottom surface ofthe above described channel layer.

According to a sixth aspect of the present invention, there is provideda method for manufacturing the compound semiconductor epitaxialsubstrate as described in the above first, second, third, fourth, orfifth aspect, characterized in that an epitaxial layer of each compoundsemiconductor is formed by the use of an MOCVD method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing a layer structure of an epitaxial substrateaccording to Example 1 of the present invention;

FIG. 2 is a drawing showing a layer structure of an epitaxial substrateaccording to Example 2 of the present invention;

FIG. 3 is a drawing showing a layer structure of an epitaxial substrateaccording to Example 3 of the present invention;

FIG. 4 is a drawing showing a layer structure of an epitaxial substrateaccording to Comparative Example 1 of the present invention;

FIG. 5 is a drawing showing a layer structure of an epitaxial substrateaccording to Comparative Example 2 of the present invention; and

FIG. 6 is a drawing showing a layer structure of an epitaxial substrateaccording to Comparative Example 3 of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

One example of the present invention will now be described in detailwith reference to the drawings. Although a layer structure of anepitaxial substrate illustrated herein by way of example represents astructure for measuring characteristics of the epitaxial substrate, alayer structure of an epitaxial substrate used for an actualpseudomorphic-HEMT further comprises other layers such as an n-GaAslayer or an n-AlGaAs layer being laminated thereon. However, it isapparent that even such epitaxial substrate used for the actualpseudomorphic-HRMT will have the same characteristics as in the case ofExamples described below.

EXAMPLE 1

FIG. 1 is a drawing for describing a cross-sectional structure ofExample 1 of a HEMT structure epitaxial substrate according to thepresent invention. In FIG. 1, a reference numeral 1 denotes a GaAs layerwhich is a crystal substrate and each of reference numerals 2 to 5denotes a buffer layer formed on the GaAs layer 1. The buffer layers 2to 5 herein are formed as an i-GaAs layer having a thickness of 200 nm,.an i-Al_(0.25)Ga_(0.75)As layer having a thickness of 250 nm, an i-GaAslayer having a thickness of 250 nm, and an i-Al_(0.20)Ga_(0.80)As layerhaving a thickness of 200 nm, respectively.

Reference numeral 6 denotes a back side electron supplying layer formedas an n-Al_(0.20)Ga_(0.80)As layer having a thickness of 4 nm and dopedwith n-type impurities at a concentration of 3×10¹⁸/cm³. A back sidespacer layers 7 and 8 are formed in this order on the back side electronsupplying layer 6. The back side spacer layer 7 herein is ani-Al_(0.20)Ga_(0.80)As layer having a thickness of 3 nm and the backside spacer 8 is an i-GaAs layer having a thickness of 5.5 nm. Referencenumeral 9 denotes a channel layer in which two-dimensional electrongases are formed in order to flow two-dimensional electronstherethrough, and is an i-In_(0.30)Ga_(0.70)As layer having a thicknessof 7.5 nm.

Each of reference numerals 10 and 11 denotes a front side spacer layer.The front side spacer layer 10 is formed as an i-GaAs layer having athickness of 5.5 nm, and the front side spacer layer 11 is formed as ani-Al_(0.20)Ga_(0.80)As layer having a thickness of 3 nm.

Reference numeral 12 denotes a front side electron supplying layerformed as an n-In_(0.483)Ga_(0.517)P layer having a thickness of 6 nmand doped with n-type impurities at a concentration of 4×10¹⁸/cm³.Reference numeral 13 denotes an undoped layer which is formed as ani-Al_(0.20)Ga_(0.80)As layer having a thickness of 39.5 nm.

Then, a method for manufacturing the epitaxial substrate having such alayer structure as shown in FIG. 1 will be described. First, a surfaceof a highly resistive semi-insulating GaAs single crystal substrate isdegreased and washed, etched, rinsed, and dried, and subsequently thissingle crystal substrate is placed on an heating table of a crystalgrowth reactor as a GaAs substrate 1.

An AlGaAs layer, an InGaAs layer, an InGaP layer and the like to beformed on the GaAs substrate 1 is vapor-phase grown by the use of anMOCVD method as described below. An inside of the reactor issubstantially substituted with high purity hydrogen before starting aheating operation, and then once a moderate and stable temperature isreached within the reactor, arsenic source materials are introduced intothe reactor and subsequently gallium source materials are introducedthereto for growing an GaAs layer. In addition, aluminum sourcematerials are introduced thereto when an AlGaAs layer is grown, andindium source materials are also introduced thereto when an InGaP layeris grown. Also, phosphorous source materials are substituted for any ofmaterials described above when an InGaP layer is grown. The desiredlaminated structure is being grown by controlling the predetermined timeand the supply of respective source materials. Finally, the supply ofrespective source materials are terminated to stop the crystal growth,and after the cooling operation, the epitaxial substrate formed bylaminating as described above is removed from the reactor to completethe crystal growth. A substrate temperature at a time of growingcrystals is usually from about 500° C. to 800° C.

As a high resistive semi-insulating GaAs single crystal substrate in thepresent invention, it is possible to use a GaAs substrate manufacturedby a LEC (Liquid Encapsulated Czochralski) method, a VB (VerticalBridgeman) method, a VGF (Vertical Gradient Freezing) method and thelike, the GaAs substrate having an angle of inclination from about 0.05°to 10° with respect to one crystallographic plane direction.

It is preferable that metalorganic compounds and/or hydrides are used assource materials during the epitaxial growth. Although arsenictrihydride (arsine) is generally used as an arsenic source material, itis also possible to use alkyl arsine which is obtained by substitutinghydrogen of arsine with an alkyl group having 1 to 4 carbons. Althoughphosphorus trihydride (phosphine) is generally used as an phosphorussource material, it is also possible to use alkyl phosphine which isobtained by substituting hydrogen of pshosphine with an alkyl grouphaving 1 to 4 carbons. As gallium, aluminum, and indium sourcematerials, it is generally used a trialkylate or a trihydride which isobtained by bonding an alkyl group having 1 to 3 carbons or hydrogen toeach metal atom.

As an n-type dopant, it is possible to use a hydride or an alkylatehaving an alkyl group with 1 to 3 carbon atoms of silicon, germanium,tin, sulfur, selenium or the like or an alkylate having an alkyl groupwhose carbon number is 1 to 3.

The epitaxial substrate shown in FIG. 1 was specifically manufactured asdescribed below. However, the present invention is not intended to belimited to this one example.

A laminated structure shown in FIG. 1 was epitaxially grown on asemi-insulating GaAs substrate in accordance with a VGF method by theuse of a low pressure barrel-typed MOCVD reactor. Trimethyl gallium(TMG), trimethyl aluminum (TMA), and trimethyl indium (TMI) were used asthe third-group elements, while arsine (AsH₃) and phosphine (PH₃) wereused as the fifth-group elements. Disilane (Si₂H₆) was used as n-typedopants. High purity hydrogen was used as a carrier gas for the sourcematerials, and the epitaxial growth was performed under the conditionsthat a pressure within the reactor was 0.1 atm, a growth temperature was650° C., and a growth rate was 3 to 1 μm/hr.

The InGaP layer in the laminated structure was epitaxially grown byadjusting an In composition so as to realize the lattice matchingbetween the GaAs layer and the AlGaAs layer. In Example 1, the Incomposition of the InGaP layer was 0.483. Also in the structure ofExample 1, the InGaP layer was epitaxially grown to obtain only a frontside electron supplying layer.

The channel layer 9 through which electrons run was epitaxially grownusing a strain InGaAs layer having an In composition of 0.30 and athickness of 7.5 nm.

Each of i-GaAs layers acting as the front side spacer layer 10 and theback side spacer layer 8 respectively were epitaxially grown to athickness of 5.5 nm so as to contact with a top surface and a bottomsurface of the InGaAs layer grown as the channel layer 9 respectively.

According to the laminated structure of Example 1 shown in FIG. 1 whichwas fabricated by the epitaxial growth as described above, the result ofperforming the hall measurement in accordance with a Van der Pauw methodshowed better measurement values which had never been obtained before,that is, the channel layer 9 had a two-dimensional electron gasconcentration of 1.81×10¹²/cm² at room temperature (300 K), an electronmobility of 8360 cm²/V·s at room temperature (300 K), a two-dimensionalelectron gas concentration of 2.13×10¹²/cm² at 77 K, and an electronmobility of 33900 cm²/V·s at 77 K. In addition, as a result ofperforming a CV measurement by using an Al schottky electrode withrespect to this structure, a pinch-off voltage at a residual carrierconcentration of 1×10¹⁵/cm³ was −1.74 V.

Since the epitaxial substrate shown in FIG. 1 is formed as describedabove, electrons are supplied from the back side electron supplyinglayer 6 to the channel layer 9 via the back side spacer layers 7 and 8,as well as supplying electrons from the front side electron supplyinglayer 12 to the channel layer 9 via the front side spacer layers 11 and10. Consequently, two-dimensional electron gases having high mobilitiesare formed on a front side and a back side of the channel layer 9. Sincethe channel layer 9 herein was grown so as to have an In composition of0.3 which is higher than 0.25, a difference can be made between aposition of a conduction band of the energy band of the channel layer 9and that of the front side electron supplying layer 12, thetwo-dimensional electron gas concentration in the channel layer 9 can beincreased, and, a two-dimensional mobility of electrons of thetwo-dimensional electron gas can be markedly improved than ever before.As a result of reviewing the above described experiments, it has beenfound that a higher two-dimensional electron mobility can be obtainedwhich exceeds a value obtained in an n-InGaP electron supplying layerHEMT structure reported heretofore, by adjusting the In composition to0.25 or more.

Indeed, as for the epiaxial substrate shown in FIG. 1, a concentrationof the two-dimensional electron gas could be increased as well as anelectron mobility within the channel layer 9 could be improved to 8000cm²/V·s or more by adjusting the In composition in the channel layer 9to 0.25 or more.

In addition, as a result of various experiments, it was confirmed thatan electron mobility in the channel layer 9 of a structure shown in FIG.1 at room temperature (300 K) could be at 8000 cm²/V·s, provided that aGaAs layer thickness of each of the back side spacer layer 8 and thefront side spacer layer 10 which are respectively in contact with abottom surface and a top surface of the channel layer 9 is 4 nm or more.

Thus, it can be considered that principal factors for improving anelectron mobility in the channel layer 9 are an In composition of thechannel layer 9 and a thickness of this layer, and further it has beenfound that the electron mobility can also be improved by using a VGFsubstrate or a VB substrate as a substrate.

EXAMPLE 2

An epitaxial substrate having a layer structure shown in FIG. 2 wasfabricated in accordance with a MOCVD method as in the case ofExample 1. In the epitaxial substrate shown in FIG. 2, a referencenumeral 21 denotes a semi-insulating GaAs substrate, reference numerals22 to 25 denote buffer layers, a reference numeral 26 denotes a backside electron supplying layer, reference numerals 27 and 28 denote backside spacer layers, a reference numeral 29 denotes a channel layer,reference numerals 30 and 31 denote front side spacer layers, areference numeral 32 denotes a front side electron supplying layer, anda reference numeral 33 denotes an undoped AlGaAs layer. Composition andthickness of respective layers are as shown in FIG. 2.

As is evident from comparing FIG. 1 with FIG. 2, Example 2 is differentfrom Example 1 because a front side spacer layer 31 is formed byobtaining an i-InGaP layer having an In composition of 0.483 and athickness of 3 nm. Formation of other layers are the same as in the caseof Example 1, respectively.

As for the epitaxial substrate obtained as described above, the resultof performing the hall measurement in accordance with a Van der Pauwmethod showed better measurement values which had never been obtainedbefore, that is, the channel layer 29 had a two-dimensional electron gasconcentration of 1.89×10¹²/cm² at room temperature (300 K), an electronmobility of 8630 cm²/V·s at room temperature (300 K), a two-dimensionalelectron gas concentration of 2.16×10¹²/cm² at 77 K, and an electronmobility of 32000 cm²/V·s at 77 K. In addition, as a result ofperforming a CV measurement by using an Al schottky electrode withrespect to this structure, a pinch-off voltage at a residual carrierconcentration of 1×10¹⁵/cm³ was −1.80 V.

EXAMPLE 3

An epitaxial substrate having a layer structure shown in FIG. 3 wasfabricated in accordance with a MOCVD method as in the case ofExample 1. In the epitaxial substrate shown in FIG. 3, a referencenumeral 41 denotes a semi-insulating GaAs substrate, reference numerals42 to 45 denote buffer layers, a reference numeral 46 denotes a backside electron supplying layer, reference numerals 47 and 48 denote backside spacer layers, a reference numeral 49 denotes a channel layer,reference numerals 50 and 51 denote front side spacer layers, areference numeral 52 denotes a front side electron supplying layer, anda reference numeral 53 denotes an undoped AlGaAs layer. Composition andthickness of respective layers are as shown in FIG. 2.

As is evident from comparing FIG. 1 with FIG. 3, Example 3 is differentfrom Example 1 because each of a front side spacer layer 51 and a backside spacer layer 47 and a back side electron supplying layer 46 isformed by obtaining an i-InGaP layer having an In composition of 0.483and a thickness of 3 nm. Formation of other layers are the same as inthe case of Example 1, respectively.

As for the epitaxial substrate obtained as described above, the resultof performing the hall measurement in accordance with a Van der Pauwmethod showed better measurement values which had never been obtainedbefore, that is, the channel layer 49 had a two-dimensional electron gasconcentration of 1.89×10¹²/cm² at room temperature (300 K), an electronmobility of 8010 cm²/V·s at room temperature (300 K), a two-dimensionalelectron gas concentration of 2.12×10¹²/cm² at 77 K, and an electronmobility of 34200 cm²/V·s at 77 K. In addition, as a result ofperforming a CV measurement by using an Al schottky electrode withrespect to this structure, a pinch-off voltage at a residual carrierconcentration of 1×10¹⁵/cm³ was −2.20 V.

COMPARATIVE EXAMPLE 1

An epitaxial substrate having a structure shown in FIG. 4 was fabricatedas Comparative Example 1 in accordance with a MOCVD method as is thecase of Example 1, except only for making modifications to the Incomposition and the thickness of the InGaAs layer used for the channellayer 9 and to the thickness of the i-GaAs layers 10 and 8 laminated onthe top and bottom surfaces of the channel layer 9 of the pseudomorphicHEMT structure epitaxial substrate of Example 1 as shown in FIG. 1. Inthe epitaxial substrate shown in FIG. 4, a reference numeral 61 denotesa semi-insulating GaAs substrate, reference numerals 62 to 65 denotebuffer layers, a reference numeral 66 denotes a back side electronsupplying layer, reference numerals 67 and 68 denote back side spacerlayers, a reference numeral 69 denotes a channel layer, referencenumerals 70 and 71 denote front side spacer layers, a reference numeral72 denotes a front side electron supplying layer, and a referencenumeral 73 denotes an undoped AlGaAs layer. Composition and thickness ofrespective layers are as shown in FIG. 4.

In Comparative Example 1 as shown in FIG. 4, an InGaAs layer used as thechannel layer 69 was prepared so as to have an In composition of 0.19and a thickness of 14.0 nm, and then i-GaAs layers to be used as theback side spacer layer 68 and the front side spacer layer 70respectively were epitaxially grown to a thickness of 2.0 nm on the topand bottom surfaces of the channel layer 69. A structure of thisComparative Example 1 is a pseudomorphic HEMT structure which hasconventionally been well known. The structure of this ComparativeExample 1 was fabricated by epitaxially growing respective layers underthe same growth condition as in the case of Example 1.

As for the epitaxial substrate of Comparative Example 1, the result ofperforming the hall measurement in accordance with a Van der Pauw methodshowed that the measurement values were almost the same as values whichhad been reported before, that is, the channel layer 69 had atwo-dimensional electron gas concentration of 1.77E12/cm²at roomtemperature (300 K), an electron mobility of 7100 cm²/V·s at roomtemperature (300 K), a two-dimensional electron gas concentration of2.06E12/cm² at 77 K, and an electron mobility of 22500 cm²/V·s at 77 K.In addition, as a result of performing a CV measurement by using an Alschottky electrode with respect to this structure, a pinch-off voltageat a residual carrier concentration of 1E15 cm³ was −1.72 V.

COMPARATIVE EXAMPLE 2

An epitaxial substrate having a structure shown in FIG. 5 was fabricatedas Comparative Example 2 in accordance with a MOCVD method as is thecase of Example 2, except only for making modifications to the Incomposition and the thickness of the InGaAs layer used for the channellayer 29 and to the thickness of the i-GaAs layers 28 and 30 laminatedon the top and bottom surfaces of the channel layer 29 of thepseudomorphic HEMT structure epitaxial substrate of Example 2 as shownin FIG. 2. In the epitaxial substrate shown in FIG. 5, a referencenumeral 81 denotes a semi-insulating GaAs substrate, reference numerals82 to 85 denote buffer layers, a reference numeral 86 denotes a backside electron supplying layer, reference numerals 87 and 88 denote backside spacer layers, a reference numeral 89 denotes a channel layer,reference numerals 90 and 91 denote front side spacer layers, areference numeral 92 denotes a front side electron supplying layer, anda reference numeral 93 denotes an undoped AlGaAs layer. Composition andthickness of respective layers are as shown in FIG. 5.

In Comparative Example 2 as shown in FIG. 5, an InGaAs layer used as thechannel layer 89 was prepared so as to have an In composition of 0.19and a thickness of 14.0 nm, and then i-GaAs layers to be used as theback side spacer layer 88 and the front side spacer layer 90respectively were epitaxially grown to a thickness of 2.0 nm on the topand bottom surfaces of the channel layer 69. The structure of thisComparative Example 2 was fabricated by epitaxially growing respectivelayers under the same growth condition as in the case of Example 2.

As for the epitaxial substrate of Comparative Example 1, the result ofperforming the hall measurement in accordance with a Van der Pauw methodshowed that the measurement values were almost the same as values whichhad been reported before, that is, a two-dimensional electron gasconcentration at room temperature (300 K) was 1.85E12/cm², an electronmobility at room temperature (300 K) was 7030 cm²/V·s, a two-dimensionalelectron gas concentration at 77 K was 2.19E12/cm², and an electronmobility at 77 K was 20800 cm²/V·s. In addition, as a result ofperforming a CV measurement by using an Al schottky electrode withrespect to this structure, a pinch-off voltage at a residual carrierconcentration of 1E15 cm³ was −1.80 V.

COMPARATIVE EXAMPLE 3

An epitaxial substrate having a structure shown in FIG. 6 was fabricatedas Comparative Example 3 in accordance with a MOCVD method as is thecase of Example 3, except only for making modifications to the Incomposition and the thickness of the InGaAs layer used for the channellayer 49 and to the thickness of the i-GaAs layers 48 and 50 laminatedon the top and bottom surfaces of the channel layer 49 of thepseudomorphic HEMT structure epitaxial substrate of Example 3 as shownin FIG. 3. In the epitaxial substrate shown in FIG. 6, a referencenumeral 101 denotes a semi-insulating GaAs substrate, reference numerals102 to 105 denote buffer layers, a reference numeral 106 denotes a backside electron supplying layer, reference numerals 107 and 108 denoteback side spacer layers, a reference numeral 109 denotes a channellayer, reference numerals 110 and 111 denote front side spacer layers, areference numeral 112 denotes a front side electron supplying layer, anda reference numeral 113 denotes an undoped AlGaAs layer. Composition andthickness of respective layers are as shown in FIG. 6.

In Comparative Example 3 as shown in FIG. 6, an InGaAs layer used as thechannel layer 109 was prepared so as to have an In composition of 0.19and a thickness of 14.0 nm, and then i-GaAs layers to be used as theback side spacer layer 108 and the front side spacer layer 110respectively were epitaxially grown to a thickness of 2.0 nm on the topand bottom surfaces of the channel layer 109. The structure of thisComparative Example 3 was fabricated by epitaxially growing respectivelayers under the same growth condition as in the case of Example 3.

As for the epitaxial substrate of Comparative Example 3, the result ofperforming the hall measurement in accordance with a Van der Pauw methodshowed that the lower measurement values could only be obtained, thatis, the channel layer 109 had a two-dimensional electron gasconcentration of 1.99E12/cm² at room temperature (300 K), an electronmobility of 5620 cm²/V·s at room temperature (300 K), a two-dimensionalelectron gas concentration of 2.16E12/cm² at 77 K, and an electronmobility of 13900 cm²/V·s at 77 K. In addition, as a result ofperforming a CV measurement by using an Al schottky electrode withrespect to this structure, a pinch-off voltage at a residual carrierconcentration of 1E15 cm³was −2.19 V.

As has been described above, the present invention is to provide aconsiderable amount of benefit even when the present invention isapplied to HEMTs, in that possibilities are open for using apseudomorphic HEMT formed on a GaAs substrate comprising InGaP electronsupplying layers and InGaP spacer layers being favorable at a time offabricating electronic devices for the purpose of obtaining varioushigh-speed devices driven within an ultra high frequency band aboveseveral tens of GHz controlled by an electron velocity being closelycorrelating with an electron mobility.

INDUSTRIAL APPLICABILITY

According to the present invention, there is provided a pseudomorphic(pseudomorphic high electron mobility field effect transistor) structureepitaxial that has favorable characteristics which have never beenreported before and is advantageous to the fabrication of electronicdevices, as described above.

1. A compound semiconductor epitaxial substrate for use in apseudomorphic high electron mobility field effect transistor, comprisingan InGaAs layer as a channel layer and an InGaP layer containing n-typeimpurities as a front side electron supplying layer, said InGaAs layerhaving an electron mobility at room temperature (300 K) of 8000 cm²/V·sor more.
 2. The compound semiconductor epitaxial substrate according toclaim 1, further comprising an InGaP layer as a front side spacer layerbetween said channel layer and said front side electron supplying layer.3. The compound semiconductor epitaxial substrate according to claim 2,further comprising an InGaP layer containing n-type impurities also as aback side electron supplying layer and comprising an InGaP layer as aback side spacer layer between said channel layer and said back sideelectron supplying layer.
 4. The compound semiconductor epitaxialsubstrate according to claim 1, 2, or 3, wherein an In composition ofthe InGaAs layer constituting of said channel layer is 0.25 or more. 5.The compound semiconductor epitaxial substrate according to claim 1, 2,or 3, wherein GaAs layers each of which has a thickness of 4 nm or moreare laminated on said channel layer in contact with a top surface and abottom surface of said channel layer, respectively.
 6. A method formanufacturing the compound semiconductor epitaxial substrate accordingto claim 1, 2, or 3, characterized in that an epitaxial layer of eachcompound semiconductor is formed by employing an MOCVD method.
 7. Amethod for manufacturing the compound semiconductor epitaxial substrateaccording to claim 4, characterized in that an epitaxial layer of eachcompound semiconductor is formed by employing an MOCVD method.
 8. Amethod for manufacturing the compound semiconductor epitaxial substrateaccording to claim 5, characterized in that an epitaxial layer of eachcompound semiconductor is formed by employing an MOCVD method.